Generally, when a hot rolled steel sheet is produced by means of a hot finish rolling mill, rolls are deflected due to rolling load, thereby sheet thickness at a central portion in sheet width becomes thicker than sheet thickness at portions in the vicinity of opposite edges of the rolled sheet, that is a sheet crown is formed in the rolled sheet. By the way, the sheet crown, if the sheet crown becomes large, makes it difficult to provide an adequate sheet profile in cold rolling in the next step, which also provides deficiency in the shape and unavoidably results in reduction in yield, so that it is required for the hot finish rolling mill to make the sheet crown as small as possible.
Thus, for a purpose of controlling the shape of sheet to reduce the sheet crown, for example, JP-A-62-10722 discloses a six high rolling mill to be installed in a post-stage stand, wherein a rolling mill array includes intermediate rolls having a constant diameter over the full length thereof arranged between backup rolls and work rolls, respectively, and these intermediate rolls are adapted to shift in the mutually opposite axial direction, thereby the ability to control the sheet crown is enhanced. Furthermore, JP-A-57-91807 discloses a rolling mill in which an S-shaped crown is formed on any one of a work roll, an intermediate roll or a backup roll, and the roll having the S-shaped crown is shifted in the axial direction, thereby the ability for controlling the sheet crown is enhanced.
However, in the former prior art disclosed in JP-A-62-10722, the length of the intermediate roll is made approximately the same as each length of the backup roll and the work roll, so that when the intermediate roll is shifted in order to make the sheet crown small, the length of contact of the intermediate roll with the backup roll and the work roll becomes short, and the mill rigidity of the rolling mill decreases, and hence, there has been such a problem that when the rolling load changes due to temperature deviation in the sheet bar or the like, the roll gap between a pair of work rolls greatly changes, and no predetermined accuracy in the sheet thickness can be provided, and there has been such a problem that when the center in sheet width deviates from the center of the rolling mill due to deviation of the sheet bar or the like, meanderings resulting from the difference in rigidity of right and left portions of the rolling mill take place, sometimes it is fallen into impossibility of rolling from occurring of reduction ears caused by miss rolling.
In addition, there has been such another problem that spalling occurs on the surfaces of rolls resulting from the increase in pressure between rolls on account of the short length of contact of the intermediate roll, and the service life of the rolls decrease.
It is noted that the problem mentioned above can be avoided by decreasing the shift amount of the intermediate rolls, but the ability for controlling the crown of the work rolls in the rolling mill is greatly limited.
And also in the later prior art disclosed in JP-A-57-91807, there has been such a problem, when the profile control is performed by shifting intermediate rolls provided with an S-shaped crown, the control of crown becomes impossible due to the abrasion of rolls.
Furthermore, when the profile control is performed by producing a curved roll crown on the intermediate roll or the backup roll, it becomes necessary to enlarge the roll crown in order to ensure a large control amount for the crown, but when a sheet bar having a relatively narrow width is rolled with small rolling load by providing such a large roll crown, non-contact portions are generated between the backup roll and the intermediate roll or between the backup roll and the work roll, and the mill rigidity of the rolling mill becomes low, which unavoidably results in the decrease in accuracy of the sheet thickness. In addition, there has been another problem that when the non-contact portions are generated, meander and reduction ears occur in the rolled sheet as a result of a difference of rigidity in the axial direction of the rolls and as a result sometimes rolling of sheet becomes impossible.